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<a name="align.vocabulary"></a><a class="link" href="vocabulary.html" title="Vocabulary">Vocabulary</a>
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<div class="toc"><dl class="toc"><dt><span class="section"><a href="vocabulary.html#align.vocabulary.3_11___basic_align__">3.11 [basic.align]</a></span></dt></dl></div>
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<div class="titlepage"><div><div><h3 class="title">
<a name="align.vocabulary.3_11___basic_align__"></a><a class="link" href="vocabulary.html#align.vocabulary.3_11___basic_align__" title="3.11 [basic.align]">3.11 [basic.align]</a>
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<p>
        Object types have <span class="bold"><strong>alignment requirements</strong></span>
        which place restrictions on the addresses at which an object of that type
        may be allocated. An <span class="bold"><strong>alignment</strong></span> is an implementation-defined
        integer value representing the number of bytes between successive addresses
        at which a given object can be allocated. An object type imposes an alignment
        requirement on every object of that type; stricter alignment can be requested
        using the alignment specifier.
      </p>
<p>
        A <span class="bold"><strong>fundamental alignment</strong></span> is represented by
        an alignment less than or equal to the greatest alignment supported by the
        implementation in all contexts, which is equal to <code class="computeroutput"><span class="keyword">alignof</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">max_align_t</span><span class="special">)</span></code>.
        The alignment required for a type might be different when it is used as the
        type of a complete object and when it is used as the type of a subobject.
      </p>
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</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{</span> <span class="keyword">long</span> <span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span> <span class="special">};</span>
<span class="keyword">struct</span> <span class="identifier">D</span> <span class="special">:</span> <span class="keyword">virtual</span> <span class="identifier">B</span> <span class="special">{</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">;</span> <span class="special">};</span>
</pre>
<p>
          When <code class="computeroutput"><span class="identifier">D</span></code> is the type of a
          complete object, it will have a subobject of type <code class="computeroutput"><span class="identifier">B</span></code>,
          so it must be aligned appropriately for a <code class="computeroutput"><span class="keyword">long</span>
          <span class="keyword">double</span></code>. If <code class="computeroutput"><span class="identifier">D</span></code>
          appears as a subobject of another object that also has <code class="computeroutput"><span class="identifier">B</span></code>
          as a virtual base class, the <code class="computeroutput"><span class="identifier">B</span></code>
          subobject might be part of a different subobject, reducing the alignment
          requirementson the <code class="computeroutput"><span class="identifier">D</span></code> subobject.
        </p>
</td></tr>
</table></div>
<p>
        The result of the <code class="computeroutput"><span class="keyword">alignof</span></code> operator
        reflects the alignment requirement of the type in the complete-object case.
      </p>
<p>
        An <span class="bold"><strong>extended alignment</strong></span> is represented by
        an alignment greater than <code class="computeroutput"><span class="keyword">alignof</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">max_align_t</span><span class="special">)</span></code>.
        It is implementation-defined whether any extended alignments are supported
        and the contexts in which they are supported. A type having an extended alignment
        requirement is an <span class="bold"><strong>over-aligned type</strong></span>.
      </p>
<div class="note"><table border="0" summary="Note">
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<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          Every over-aligned type is or contains a class type to which extended alignment
          applies (possibly through a non-static data member).
        </p></td></tr>
</table></div>
<p>
        Alignments are represented as values of the type <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code>.
        Valid alignments include only those values returned by an <code class="computeroutput"><span class="keyword">alignof</span></code>
        expression for the fundamental types plus an additional implementation-defined
        set of values, which may be empty. Every alignment value shall be a non-negative
        integral power of two.
      </p>
<p>
        Alignments have an order from <span class="bold"><strong>weaker</strong></span> to
        <span class="bold"><strong>stronger</strong></span> or <span class="bold"><strong>stricter</strong></span>
        alignments. Stricter alignments have larger alignment values. An address
        that satisfies an alignment requirement also satisfies any weaker valid alignment
        requirement.
      </p>
<p>
        The alignment requirement of a complete type can be queried using an <code class="computeroutput"><span class="keyword">alignof</span></code> expression. Furthermore, the types
        <code class="computeroutput"><span class="keyword">char</span></code>, <code class="computeroutput"><span class="keyword">signed</span>
        <span class="keyword">char</span></code>, and <code class="computeroutput"><span class="keyword">unsigned</span>
        <span class="keyword">char</span></code> shall have the weakest alignment
        requirement.
      </p>
<div class="note"><table border="0" summary="Note">
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<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          This enables the character types to be used as the underlying type for
          an aligned memory area.
        </p></td></tr>
</table></div>
<p>
        Comparing alignments is meaningful and provides the obvious results:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            Two alignments are equal when their numeric values are equal.
          </li>
<li class="listitem">
            Two alignments are different when their numeric values are not equal.
          </li>
<li class="listitem">
            When an alignment is larger than another it represents a stricter alignment.
          </li>
</ul></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          The runtime pointer alignment function can be used to obtain an aligned
          pointer within a buffer; the aligned-storage templates in the library can
          be used to obtain aligned storage.
        </p></td></tr>
</table></div>
<p>
        If a request for a specific extended alignment in a specific context is not
        supported by an implementation, the program is ill-formed. Additionally,
        a request for runtime allocation of dynamic storage for which the requested
        alignment cannot be honored shall be treated as an allocation failure.
      </p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2014 Glen Fernandes<p>
        Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost
        Software License, Version 1.0</a>.
      </p>
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